advertisement 4-Phase Monolithic Synchronous Boost Converter Delivers 2.5A with Output Disconnect in a 5mm × 5mm QFN Package Design Note 332 David Salerno Introduction The LTC®3425 is the industry’s first 4-phase, monolithic synchronous boost converter. It can start up with as little as 1V and operate with inputs up to 4.5V. The output voltage range is 2.4V to 5.25V, making it well suited for battery-powered applications as well as point-ofload regulation in low voltage systems. The 4-phase architecture allows for an effective switching frequency of up to 8MHz, which in turn reduces output ripple current and peak inductor current by a factor of four (over an equivalent single-phase circuit). This allows the use of small, low cost, low profile inductors and ceramic capacitors even at high load currents. With 5A peak current capability and an effective switch RDS(ON) of 40mΩ (NMOS) and 50mΩ (PMOS), the LTC3425 is capable of efficiently delivering 2.5A load current from a 0.8mm maximum profile, 32-pin 5mm × 5mm QFN package. Multiple Operating Modes Optimize Performance in Different Applications The LTC3425 can be configured for automatic Burst Mode® operation, fixed frequency mode with forced continuous conduction or fixed frequency mode with pulse skipping. Programmable automatic Burst Mode operation is ideal for portable applications, where the load current can vary over a wide range and efficiency is paramount. A quiescent current of just 12μA in Integrated output disconnect allows VOUT to go to 0V in shutdown while eliminating the high inrush current typical of traditional boost converters during start-up. VIN 2V TO 3V L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. CIN 1μF RLIM 75k RT 15k GND 32 CSS 0.047μF 1 2 L1 2.7μH 3 4 VOUT 5 L2 2.7μH 6 7 8 31 SS SHDN GNDA 30 SYNCIN 29 VIN 28 RT 27 ILIM 26 COM 25 SYNCOUT GNDD GNDA GNDD SWA SWD VOUTA VOUTD LTC3425 VOUTB VOUTC SWB SWC GNDB GNDC GNDB GNDC REFEN VOUTS SGND 9 10 11 CFF 22pF 23 22 L4 2.7μH VOUT 3.3V 2.5A 21 20 19 L3 2.7μH R1 100k 18 17 FB COMP BURST REFOUT PGOOD 12 13 14 15 16 PGOOD CZ 330pF RF2 1M RFF 10k 24 RF1 590k RC 47k CP 15pF CB 0.022μF RB 27k CBULK 47μF DN332 F01 CBULK: TDK C3225X5R0J476M CIN: TDK C1608X5R1A105M COUT: TDK C2012X5ROJ106M ¥4 L1 TO L4: TDK RLF5018T-2R7M1R8 Figure 1. 2-Cell to 3.3V/2.5A Boost Converter 03/04/332_conv COUT 10μF w4 GND Burst Mode operation extends battery life during light load operation. For noise-sensitive applications fixed frequency mode can be selected, with either forced conduction for low noise at light load or pulse skipping for improved light load efficiency. Quiescent current in shutdown is less than 1μA. Fault Protection The LTC3425 includes short-circuit protection, programmable peak current limit and thermal shutdown. High Power and High Efficiency in a Small Package Figure 1 shows an LTC3425 application using all ceramic capacitors and low profile inductors to deliver 2.5A load current at 3.3V from a 2-cell input (VIN of 2V to 3V). Maximum component profile is only 2.5mm. In this example, the oscillator frequency is programmed for 4MHz (1MHz/phase), resulting in 10mVP-P output ripple at full load (see Figure 2). The burst resistor is selected to automatically transition from Burst Mode operation to fixed frequency mode when the load exceeds 100mA for optimal efficiency (peaking at 96%) over the load range (see Figure 3). No-load input current is only 39μA at 2.4V. The current mode architecture results in excellent transient response during a load step (see Figure 4). The use of a small feed-forward RC network across the top feedback resistor provides improved transient response and reduced output ripple in Burst Mode operation, especially when using low output capacitor values. The feed-forward capacitor also compensates for the effect of stray capacitance at the FB pin when using large value feedback resistors. The open-drain PGOOD output goes low when VOUT is 11.5% below its regulated value and goes high when VOUT is within 9% of its regulated value. Conclusion The LTC3425’s multiphase architecture makes it possible to deliver high power in a small, low profile package with very low output ripple. It offers features that are demanded in high performance portable applications, including output disconnect, automatic Burst Mode and high efficiency over a wide load range. It also offers flexible design features, including its programmable soft-start, current limit and oscillator frequency, external compensation and multiple available operating modes. VOUT 100mV/DIV AC COUPLED ILOAD 0.5A/DIV 0A Figure 2. Output Voltage Ripple at 2.5A Load for Converter of Figure 1 100 DN332 F04 Figure 4. Response of the Circuit in Figure 1 to a 1A Load Step VIN = 3V 90 VIN = 2V 80 EFFICIENCY (%) VIN = 2.4V 100μs/DIV VOUT = 3.3V ILOAD = 0.5A TO 1.5A VIN = 2.4V 70 60 50 40 30 20 10 0 0.1 1 10 100 1k LOAD CURRENT (mA) 10k Figure 5. The LTC3425 Can Deliver 10W in a Low Profile 0.7in2 Footprint DN332 F03 Figure 3. Efficiency vs Load of the Converter in Figure 1 Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn332f_conv LT/TP 0304 344K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2004